This invention generally concerns the field of reaction injection molding of polyurethanes. More specifically, it concerns internal mold release agents useful in reaction injection molding.
Reaction injection molding (RIM) is a process for producing and molding polyurethanes which process has proven especially useful for making large molded objects having resilience such as furniture items; light weight building materials; shoe soles and heels; industrial service parts such as rollers, gears, bearing pads, pump housings; auto body parts such as bumpers, fascia, hoods, doors, and fenders; and the like. RIM may be used in a one shot or a two shot process.
The RIM technique involves filling a mold with a stream of reactive, liquid starting components, which are rapidly injected using an impingement mixing process. The starting components mix by impingement as they are injected into a mixing head, from which they flow rapidly into the mold. On mixing, the components quickly begin to react to form polyurethanes. The mixture should remain fluid for a period of time sufficient to fill the mold, which is often of large volume and complex design. The period of time required to fill the mold is the filling time. The period of time between initial mixing of starting components and the first visible reaction is called cream time. The period of time between initial mixing and formation of sufficient gel to solidify the polyurethane is called gel time. Shortly after gelling, the polyurethane cures to form a molded object having sufficient dimensional stability that it can be removed from the mold, leaving an empty mold for another injection of starting components. The time from injection until such an object is formed is called the demold time. The demold time is but one component of the cycle time, which is a measure of the time from one injection until a RIM machine is ready for the next injection of starting components. Cycle time also includes time necessary for mechanical operations such as opening and closing a mold, time required for mold preparations such as applying external mold release agents, and the like.
Starting components enter the mold, gel, cure and are removed very rapidly. Typically, the mixed stream of components enters the mold at rates of from about 20 to about 1200 pounds per minute (from about 9 to about 545 kilograms per minute). Typical gel times range from less than about one second to about fifteen seconds. Cycle times are typically from about three to about five minutes and with high speed equipment are often less than about two minutes. Every stage of the process is preferably optimized to achieve cycle times which are as short as possible.
When RIM is used in a one shot process of producing polyurethanes, the polyisocyanate starting component is supplied to the mixing head in one stream and the active hydrogen starting component is supplied in another stream. In a two shot process, an isocyanate-containing prepolymer is supplied in one stream while the remaining active hydrogen component is supplied in the other. The stream of polyisocyanate or prepolymer component(s) is referred to as the "A side", or "A component" while the stream containing the active hydrogen component(s) is referred to as the "B side", or "B component." Chain extenders, catalysts, and additives are typically blended and introduced in the B side, but in certain cases can be mixed into the "A side". In some instances a third or fourth stream is required to meter an active ingredient which is incompatible with, prematurely react with or otherwise cause difficulty with one or more of the other components of the polyurethane. Polysiloxane mold release agents and pigments, for instance, are sometimes added in a third or fourth stream. Avoiding use of more than two streams is generally preferred. The term "reaction mixture" as used herein refers to an admixture of at least one polyisocyanate component and at least one hydrogen component which will form a polyurethane. Additives are optionally included in a reaction mixture.
It is important in RIM that starting components be mixed in desired proportions and that they be mixed intimately. It is also important that inclusion of air in the molded product not be in visible bubbles or pockets. The mold must be filled completely. These and other requirements are met by processes known to those skilled in the art. Such processes include processes disclosed in U.S. Pat. Nos. 3,709,640; 3,857,550; 4,218,543; 4,298,701; 4,314,962; 4,582,887, which are incorporated herein by reference. Additional description of RIM processes is found in Prepelka and Wharton, "Reaction Injection Molding in the Automotive Industry," Journal of Cellular Plastics, vol. 2, no. 2, pp. 87-98 (1975) and Knipp, "Plastics for Automobile Safety Bumpers," Journal of Cellular Plastics, pp. 76-84 No. 2 (1973).
Efficient RIM processes produce the largest possible quantity of molded products in the shortest possible time. Several factors enhance efficiency. Starting components must react quickly and harden or cure into objects which can be removed from the mold without collapsing or losing structural integrity. Speed of reaction must, however, require sufficient time that the mold can be filled before increased viscosity or gelling prevent complete filling of the mold. After the molded products are sufficiently cured to be removed from the mold, efficient processes require rapid removal from the mold. Efficient processes also avoid times in which the molds are not in use, such as maintenance time and the like.
Improvements in formulation of polyurethane components supplied to a RIM apparatus have reduced cure times to less than about 40 seconds. With such low cure times, the time required to remove a molded object from the mold and prepare the mold for the next injection of starting components has come to be one of the more time-consuming steps in a RIM process. Polyurethanes tend to adhere to a mold. Separation of the molded object from the mold is generally facilitated by coating the internal surface of a mold cavity with an agent, referred to herein as an external mold release agent, which prevents the polyurethane from sticking to the surface. Commonly used external mold release agents include waxes, soaps, oils and silicon compounds. Molds are often complex in configuration and must be completely and uniformly covered, usually by spraying a solution or emulsion of the agent onto the mold surface. Typically, this spraying is necessary after the removal of each object or each few objects from the mold. Such spraying requires labor and increases the cycle time by as much as about 50 percent. Uneven spraying often leads to sticking of molded objects in some places and accumulation of excess external mold release agent in others. In addition, repeated spraying results in excess mold release agent building up, especially on surfaces having intricate patterns, where it causes surface defects. Periodic cleaning of the mold typically increases average cycle time an additional 10 to 20 percent. Additionally, mold release agent adheres to the molded object and must be removed before paint and the like will adhere to the object.
Clearly, reducing the need for an external mold release agent could make RIM molding much more efficient. A number of materials have been added to polyurethanes to improve mold release qualities of the polymer. These materials are called internal mold release agents and frequently have a silicon-containing molecular structure, as exemplified by the compositions disclosed in U.S. Pat. Nos. 3,725,105; 3,993,606; 4,024,090; 4,076,695; 4,111,861; 4,220,727; 4,379,100; 4,396,729; and 4,546,154. Many other compositions which have been proposed for use as internal mold release agents for use in RIM include fatty acid structures as exemplified by compositions disclosed in U.S. Pat. Nos. 4,058,492; 4,111,861; 4,130,698; 4,201,847; and 4,254,228. These proposed internal mold release agents have generally found limited application because they have limited compatibility with the starting components used to form polyurethanes or react with some of the components, catalysts or additives.
Many internal mold release agents cannot, therefore, be added in the A or B component streams of a RIM process and require use of a third or fourth stream as explained in U.S. Pat. No. 4,546,154. Use of a third or fourth stream requires specialized equipment and additional material handling. Silicon-based mold release agents are also often difficult to remove from surfaces of molded objects to allow satisfactory adherence of paint and other coatings.
Improving the efficiency of RIM processes by use of an internal mold release agent which aids in removal of a molded object from the mold without substantial build up of the agent on the surfaces of the mold or of the molded object would be desirable. Such an internal mold release agent would preferably be suitable for use in a two stream RIM process.